Heretofore, a number of processes have been developed with the object of chemically producing pulp from cellulose raw materials. Many processes have been weeded out so far; the processes which are currently available for the preparation of chemical pulp are the AP method (alkali method), SP method (sulfite method), KP method (kraft method) and variants thereof.
The AP method employs a sodium hydroxide aqueous solution (consisting of two components) as a cooking liquor. This method offers the advantages that no malodorous substances are produced, unlike the KP method, and that the chemicals can be recovered from the pulp waste liquor with relative ease. However, it suffers from the disadvantages that the removal of lignin does not readily occur in the process of pulping so that the resulting pulp is poor in strength and the kappa value (an indicator of the content of lignin in pulp having the relationship: lignin (%)=kappa value.times.0.15) is so remarkably high that a large quantity of chemical is required for bleaching. Hence, this method is not usually applied to the pulping of wood and it is utilized, in part, only for pulping cellulose raw materials derived from non-wooden materials.
The SP method employs an acidic, neutral or alkaline solution of a sulfite as a cooking liquor, and the acidic SP method is particularly superior in its ability to elute lignin so that the unbleached SP pulp is low in kappa value and refining and bleaching are readily carried out; however, the strength and the yield of the pulp are poor. Hence, this method is practically applied for the preparation of dissolution pulp from needle-leaved trees and some broad-leaved trees; however, the demand for such pulp is extremely low. Further, the SP method is not suited for pulping general broad-leaved trees or those needle-leaved trees which are difficult to digest, and the treatment of pulp waste liquor and recovery of the chemical substances used are not easy, so that this method is currently applied only in an extremely small sector of the industry.
The KP method uses an aqueous solution of sodium sulfide and sodium hydroxide (consisting of three components) as a cooking liquor and can pulp various kinds of needle-leaved trees and broad-leaved trees. The resulting pulp is tough and the kappa value is relatively low; however, its bleaching is not so easy. Generally, five- to seven-step bleaching gives a bleached pulp having a high degree of whiteness. Further, this method offers the advantages that sodium sulfide and sodium hydroxide can be recovered for reuse in the cooking liquors by concentrating pulp wastes, burning them in a reducing atmosphere and subjecting them to causticization. In addition, the energy used in burning can also be recovered. For the foregoing reasons, the KP method is today generally used to a remarkably wide extent, and more than 70% of total production of pulp and more than 95% of production of chemical pulp in Japan is by the KP method.
It is to be noted, however, that recent and more severe requirements for protection of the environment and conservation of earth resources, are difficult to meet with the KP method, thus creating an increasing demand to develop a new method for the preparation of pulp, which can serve as a substitute for the KP method currently prevailing in this industry. In other words, while the KP method is superior to the other conventional methods in terms of utilization of resources of cellulose, because the KP method can pulp a wider variety of needle-leaved and broad-leaved trees, the KP method is not suited for pulping so far unavailable trees including many kinds of tropical trees, ceders, deciduous trees and the like and for bleaching pulp therefrom. Further, this method can utilize only a limited number of raw materials, i.e. it is inappropriate for pulping a large number of non-wooden materials including rice plant straw, bagasse, tow, fibers of banana and the like. In addition, the KP method causes by-production of malodorous substances including sulfurous substances such as hydrogen sulfide, methyl mercaptan and the like in exhaust gases resulting from the digestion of pulp, thereby causing air pollution. Furthermore, the bleaching of unbleached KP requires a large quantity of chlorinated bleaching chemicals so that a large amount of organic chlorinated compounds are formed and their presence in waste water from the bleaching step is a huge source of pollution. It should be further noted that, as the purity of product pulp prepared by the XP method is high, a large majority of impurites contained in the cellulose raw materials, such as silica, calcium, magnesium, iron and the like, are eluted out in the digesting step and contaminate the pulp waste liquor; however, no appropriate technique capable of separating and removing those impurities has yet been developed. Therefore, if the chemicals are recovered from the pulp waste liquor and utilized, these impurities are further concentrated, thereby leading to incapability of treating the pulp waste liquor itself. Hence, some cellulose raw materials such as rice plant straw, parts of tropical trees, which are rich in ash components, particularly in silica, can be digested by the KP method; however, the resulting waste liquor cannot be treated in an effective way so that it must be discharged without sufficient treatment. In these respects, the KP method presents many defects as a total system. Accordingly, the pulp industry has been incapable of development notwithstanding an enormous quantity of resources of cellulose that is not yet utilized and a sufficient demand of paper pulp.
Research on new methods for pulping can be broken down into three main groups: (1) methods for preparing pulp by reducing cellulose raw materials in a mechanical way by a disc refiner or the like or in combination with a light degree of chemical treatment; (2) methods for preparing pulp by decomposing non-fibrous materials in the cellulose raw materials by the aid of bacteria or by means of enzymatic treatment; and (3) methods or increasing yield of pulp by adding a small quantity of an auxiliary agent to a cooking liquor to be employed in a conventional chemical pulping method.
The methods (1) for the mechanical preparation of the pulp include the GP method. This method produces a high yield of the pulp but requires a large quantity of energy consumption. Further, the pulp prepared by this method still contains a Considerably large amount of lignin so that a large quantity of a bleaching agent is required for bleaching. The unbleached pulp is too poor in quality to be employed as is. Hence, in recent years, a variant has been adopted which comprises chemically treating the cellulose raw materials to a light extent by using the cooking liquor as employed in the AP method, the SP method, the KP method or the like, or an alkali solution of hydrogen peroxide, in combination with mechanical treatment. This variant method can provide a pulp of quality better than the GP method, in a yield higher than chemical pulp; however, it still has many problems left unsolved, in that it consumes a large quantity of electric power in the preparation of the pulp, a large amount of chlorinated bleaching agent in bleaching, treatment of pulp waste liquor and the like.
The methods (2) for isolating cellulose by biochemical means are recently being extensively studied because the pulp can be produced at ambient atmosphere and temperature or at temperatures close thereto. However, a big problem left unsolved, is that only lignin is separated and removed in an extremely short time without decomposition of cellulose by using a microorganism or an enzyme extracted therefrom.
The methods (3) attempt an increase in the yield of pulp by adding an auxiliary agent such as AQ (anthraquinone) or the like to the cooking liquor employed in the conventional methods, and it is reported that the yield of the pulp can be increased by approximately 0.5% by adding AQ in the KP method, the SP method and the AP method. However, further improvements in the yield are not easy to accomplish.
The present invention has as its object to provide a process and a comprehensive system to produce pulp of good quality in a high yield, which pulp is high in quality and in degree or whiteness and capable of being easily bleached, from a wide range of cellulose raw materials, including wooden and non-wooden materials, and to recover energy and chemicals from the waste liquor in a ready and continuous way, in order to solve the problems with natural resources and with the environment, which are now obstacles to development of the pulp industry.